This invention relates to an improved process for preparation of ethylene oxide by the partial oxidation of ethylene by means of gaseous oxygen in the presence of a silver containing catalyst. More particularly, this invention is directed to an improvement in the vapor phase oxidation of ethylene to ethylene oxide over a fixed catalyst bed of refractory support carrying active silver wherein the reaction zone effluent is immediately subject to cooling in a cooling zone filled with an inert particulate having a critically low surface area which minimizes isomerization of ethylene oxide to acetaldehyde.
The desirability of minimizing isomerization of ethylene oxide to acetaldehyde in conventional processes for direct oxidation of ethylene to ethylene oxide has long been recognized in the art. In such processes, which typically employ one or more tubular reactors containing fixed beds of silver catalyst on a refractory support, it is known that acetaldehyde is rapidly converted to carbon dioxide and water under ethylene oxide forming conditions. Further, any significant quantity of acetaldehyde not oxidized in the reaction zone appears as an unwanted impurity in the reaction product which must be rejected in downstream processing operations to meet product quality specifications. Thus, unless acetaldehyde formation is minimized, it can become a major factor contributing to ethylene oxide yield losses in the process as well as a troublesome product unpurity which increases the costs of downstream product processing and purification.
Since the high temperature conditions employed in the catalytic reaction zone appear to be particularly condusive to the isomerization of ethylene oxide to acetaldehyde and rapid combustion of the acetaldehyde formed to carbon dioxide and water, it is advantageous to effect rapid cooling of the reaction product as it leaves the reaction zone. To this end, typical commercial scale processes for catalytic oxidation of ethylene to ethylene oxide sometimes employ a post cooling section immediately downstream of the reaction zone to at least partially cool the reaction product to temperatures below those required for oxidation. This post cooling section may be packed or unpacked and is typically located adajcent to the reaction zone, very suitably as an extension to, or part of, the tubular reaction zone, itself. Since the use of packing in the post cooling section functions to reduce the residence time of the reaction zone effluent at the high temperatures required for ethylene oxide formation, there is good reason to prefer a packed cooling section over one which is unpacked. However, packings which have been used previously, including, typically, the catalyst carrier materials employed in the reaction zone, tend to promote the isomerization of ethylene oxide under the conditions which exist at the outlet of the reaction zone i.e., high temperature coupled with higher concentrations of ethylene oxide. These carrier materials, which are generally particulate refractory oxides having surface areas of at least 0.2 m.sup.2 /g, apparently possess a certain activity for ethylene oxide isomerization. As a result, the benefits obtained through reduced residence time with packing materials previously employed are substantially diminished by increases in the rate of ethylene oxide isomerization attributable to the packing material. Thus, it would be desireable if an inert packing material could be found which would give all the benefits of reduced residence time in the past cooling zone with little or no increase in the rate of ethylene oxide isomerization.